Knowledge has come to me recently that mussels can suck up pollutants, making them crucial cleanup crews in the New York filth. This knowledge is the inspiration for this alternate scenario, in which coral, sponges and crinoids (flower-like relatives of the sea star) have been extinct for 65 million years.

To make the scenario even tighter, let us focus on the waters highlit in the map above, the oceans of the Tropics. Temperate and polar waters undergo seasonal changes, which exclude them from this scenario. Western South America and southwestern Africa are also excluded because they are chilled by coldwater currents, and cold currents carry nutrients, and a cloud of nutrients clouds up sunlight.

Reefs can be found only on the edges of continental shelves, where nutrients from the deep are pushed upwards to the surface. The reefs here consist of clams, mussels, oysters, tunicates ("sea squirts"), goose barnacles, basket stars and worms belonging to Sabellidae (feather duster worms), Serpulidae (“Christmas tree” worms) and Siboglinidae (beard worms). These reefbuilders can be found in greater numbers in the temperate and polar shallows worldwide.

Between the reefs and the tidal pools, one would expect bare, sandy seabed stretching the horizon.

But not here. Instead, between the reefs and tidal pools are nothing but this:

Not seagrass meadows, as they are called back home, but seagrass savannas. You can find both habitats put together worldwide in tropical shallows between Cancer and Capricorn (excluding western South America and Namibia, of course.)

A reef consisting primarily of mollusks have been known to suck up pollutants. And seagrass, being plants, inhale carbon dioxide and exhale oxygen. Would a global union of both habitats (within the limits of 23.5 degrees latitude, of course) solve the bulk of the problems regarding ocean pollution and manmade greenhouse gases?

$\begingroup$I used to keep oysters but ocean acidification on top of radioactive fallout, I get marbles instead... I curse you everybody...$\endgroup$
– user6760Nov 30 '16 at 9:11

$\begingroup$your big problem are places were the seafloor is hard, that's where corals thrive, often only building up sediment later as the create natural baffles. hard substrates will not support seagrass. so something is going to evolve to take advantage of these open areas. then you have things like atolls.$\endgroup$
– JohnNov 30 '16 at 14:34

$\begingroup$@John Hard and far away from any nutrients. No, the reefs stay on the precipice.$\endgroup$
– JohnWDaileyNov 30 '16 at 14:38

$\begingroup$Hard and swarming with nutrients, that's what filter feeding does pull nutrients directly from the water. naked rock is not good for sequestering carbon but coral is.$\endgroup$
– JohnNov 30 '16 at 15:03

$\begingroup$@John The farther you are from currents, the harder it is to find food. It's that simple.$\endgroup$
– JohnWDaileyNov 30 '16 at 15:18

3 Answers
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Your mollusc and worm reef builders will be fine for removing carbon dioxide from the atmosphere. Reef-building animals have been doing this for millions of years - including long before scleractinian corals evolved. It is the reason that the amount of carbon dioxide in Earth's atmosphere has declined over time. See this question I asked for some figures on CO2 in various geological periods.

They'll get in trouble if humans release enough CO2 to cause the oceans to become slightly more acidic. Calcium carbonate shells start to dissolve even in really weak acid, like vinegar. In such a case, the poor mollusc will be working flat out just to repair the acid erosion to its shell. The CO2 it is capturing will be used to replace the CO2 released by the shell surface dissolving, so there will be no net carbon capture.

I'm not sure how either molluscs or seagrasses would solve pollution problems such as plastic fragments.

Plants don't reduce CO2, nor produce net oxygen. plants still have to respire as do the things that eat them. The carbon they take in is released during respiration either by them or whatever eats them. So seagrass fields will do nothing to affect CO2 concentrations unless you are periodically harvesting and burying them. Most forests are oxygen neutral as well and only sequester carbon at the rate of burial of fallen trees most of which get recycled and thus released. bogs are the areas that lock away carbon due to high burial and anoxic conditions.
http://earthobservatory.nasa.gov/Features/CarbonCycle/carbon_cycle2001.pdf

I'll break it down, plants photosynthesis during daylight hours making oxygen and sugar out of CO2 and water, and respire all day and night turning sugar and oxygen back into CO2 and water. So the only carbon they lock away (and the only net oxygen they produce) is in building their cellulose(sugar) tissue this gets rebalanced (oxy consumed CO2 released) as soon as they decay or are eaten. To get a net gain over the long run you need to sequester the carbon produced, the most common way is by burial, by locking it away in a anaerobic environment, which seagrass are not doing, or you can do what coral does and turn it into a mineral that is not getting digested.

Cyanobacteria are the things that produce huge quantities of net oxygen, becasue they mostly undergo anaerobic respiration to consume the sugars they produce, which does not consume oxygen. And when they die they tend to end up on the deep ocean floor which tends towards anoxic conditions.

Worse yet corals actually do sequester carbon, becasue the skeleton is made of calcium carbonate and it does not get recycled easily or often, unlike plant tissue. It is actually one of the most effective biological means of sequestering carbon. So you are actually slowing sequestration by not having corals. I should also mention that the symbiotic algae in coral does photosynthesize so if it was not for their skeletons they would be largely neutral as well.
http://earthobservatory.nasa.gov/Features/CarbonCycle/page2.php

Also reefs can be found in far more places than just the edges of continental shelves, atolls are a good example. Corals are not limited to the narrow locations you are limited your reefs to so you are trading carbon sequestration for either nothing or miniscule trace sequestration. This results in a net increase in atmospheric carbon across the globe.

$\begingroup$Where did you get THAT?!$\endgroup$
– JohnWDaileyNov 30 '16 at 14:38

$\begingroup$basic understanding of botany and the carbon cycle. but I will get you a source.$\endgroup$
– JohnNov 30 '16 at 14:42

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$\begingroup$sure plants then take that food and combine it with oxygen to make ATP. the net change in oxygen in the atmosphere is zero. the only carbon that is sequestered is the carbon in the plant tissue which is released as soon as they are eaten or decompose. seagrass is going to sequester less carbon than coral. you are creating the opposite of the intended effect.$\endgroup$
– JohnNov 30 '16 at 18:38

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$\begingroup$@JohnWDailey: Because plants absorb carbon while they live, but when they die and rot, or are eaten, most of it returns to the atmosphere. So to have a net reduction in atmospheric CO2, you'd have to 1) Increase the total living biomass; 2) Sequester some of the organic material as e.g. soil humus; or 3) Bury the organic material somehow.$\endgroup$
– jamesqfNov 30 '16 at 18:38

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$\begingroup$I see part of the problem, you think I mean net by if you stick a single plant in a jar by itself there will be no increase in oxygen. I said net for forest ,for the ecosystem they are part of. If you put an entire forest in jar including the animals that live there is little to no change in oxygen or carbon. you have to sequester the carbon for that to happen and most plant based ecosystems don't do a lot of that. bogs (and deep sea algae) sequester becasue when they die they fall into anaerobic environments and do not rot much and what little they do is done without oxygen.$\endgroup$
– JohnDec 2 '16 at 2:14

Many filter feeders preferentially concentrate some nasty chemicals; however transplanting species should be approached with considerable caution. And some mussel species, accidentally transported, have become serious pests, for example the Zebra Mussel:
https://en.wikipedia.org/wiki/Zebra_mussel

On the other hand, we can add rocks or similar to sandy or muddy shallows, to give sessile sea creatures anchor points against the current. Just any old junk doesn't work, however. Attempts to use old tires to foster artificial reefs haven't worked very well. Concrete and old ships seem to do much better for increasing the local biota.

$\begingroup$"Despite claims that as many as half of fish species have disappeared from areas where Caulerpa grows, scientific studies have shown that fish diversity and biomass are equal or greater in Caulerpa meadows than in seagrass beds, that Caulerpa had no effect on composition or richness of fish species, and that species richness and epiphytic plant diversity is greater in Caulerpa than in pure sea grass. Thus, [...] the species appears to have many beneficial ecological effects on aquatic communities in the Mediterranean Sea." (Wikipedia)$\endgroup$
– AlexPNov 30 '16 at 12:11